A substrate processing apparatus, which performs a plasma process on a semiconductor wafer (hereinafter, simply referred to as “wafer”) serving as a substrate, includes a chamber (processing chamber) that accommodates a wafer and can be depressurized; a susceptor positioned on the lower part within the chamber; and a shower head (upper electrode) provided within the chamber to face the susceptor. The susceptor is configured to mount thereon the wafer and serves as an electrode applying a high frequency power from a connected high frequency power supply into the chamber. The shower head introduces a processing gas into the chamber and is grounded to serve as a ground electrode. In this substrate processing apparatus, the processing gas supplied into the chamber is excited into plasma by the high frequency power and the wafer is plasma-processed by the plasma.
However, in order to appropriately distribute the plasma within the chamber, particularly, in a space between the shower head and the susceptor, there has been developed a substrate processing apparatus having a movable susceptor, thereby adjusting a thickness (hereinafter, referred to as a “gap”) of a processing space between the shower head and the susceptor (see, for example, Patent Document 1). Besides, on account of the restriction on the layout around the substrate processing apparatus, a substrate processing apparatus having a movable shower head, not the movable susceptor, is recently under development.
FIG. 12 is a cross sectional view schematically illustrating a configuration of a substrate processing apparatus having a movable shower head.
In a substrate processing apparatus 100 of FIG. 12, a shower head 103 is installed within a cylindrical chamber 101 so as to face a susceptor 102. The shower head 103 is formed into a substantially circular plate shape having an outer diameter substantially the same as an inner diameter of the chamber 101. The shower head 103 is configured to vertically move like a piston in the chamber 101 by a non-illustrated lift mechanism. Further, installed between the shower head 103 and a ceiling portion of the chamber 101 is a bellows 104 expansible and contractible along with the vertical movement of the shower head 103. The bellows 104 seals the inside of the chamber 101 from its surrounding air. Furthermore, in FIG. 12, the shower head 103 at the lowermost position is indicated by a solid line, and the shower head 103 at the uppermost position is indicated by a dashed line.
Patent Document 1: Pamphlet of International Patent Publication No. WO03/003437 (FIG. 1)
However, this substrate processing apparatus 100 is configured to keep some gap between the shower head 103 and a sidewall 101b of the chamber 101, so that the shower head 103 smoothly moves up and down, thereby preventing generation of particles caused by a friction between the shower head 103 and the sidewall 101b. That is, since the shower head 103 is not in contact with the sidewall 101b, a direct current may not be flown from the shower head 103 to the sidewall 101b and an alternating current may be hardly flown from the shower head 103 to the sidewall 101b. Accordingly, in the substrate processing apparatus 100, a high frequency current caused by a high frequency power applied to the susceptor 102 flows through the susceptor 102, the processing space, the shower head 103, the bellows 104, a ceiling wall 101a of the chamber 101, and the sidewall 101b of the chamber 101 in sequence, as depicted by the arrow in FIG. 12.
The bellows 104 is made of stainless steel for increasing durability and has higher impedance than that of the other parts (e.g., the chamber 101, the shower head 103, and the like) made of aluminum. Accordingly, a potential difference is generated along the bellows 104, to be specific, between the shower head 103 and the ceiling wall 101a of the chamber 101. Therefore, there are concerns that an electric field may be generated in a space (hereinafter, referred to as “upper space”) US between the shower head 103 and the ceiling wall 101a. 
This electric field ionizes a processing gas introduced into the upper space US from the gap, so that plasma is generated. However, there is a problem in that the plasma generated in the upper space US erodes wall surfaces of the chamber 101 or the shower head 103, and forms a deposit.